This invention relates to a counter-flow asphalt plant used to produce a variety of asphalt compositions. More specifically, this invention relates to a counter-flow asphalt plant having a recycle asphalt (RAP) feed to the combustion zone to produce high percentage RAP mixes without generating excessive blue smoke and having an exhaust gas heater to improve production rates with greater economy and efficiency of plant design and operation.
Several techniques and numerous equipment arrangements for the preparation of asphaltic cement, also referred by the trade as xe2x80x9chotmixxe2x80x9d or xe2x80x9cHMAxe2x80x9d are known from the prior art. Particularly relevant to the present invention is the continuous production of asphalt compositions in a drum mixer asphalt plant. Typically, water-laden virgin aggregates are dried and heated within a rotating, open-ended drum mixer through radiant, convective and conductive heat transfer from a stream of hot gases produced by a burner flame. As the heated virgin aggregate flows through the drum mixer, it is combined with liquid asphalt and mineral binder to produce an asphaltic composition as the desired end-product. Optionally, prior to mixing the virgin aggregate and liquid asphalt, reclaimed or recycled asphalt pavement (RAP) may be added once it is crushed up or ground to a suitable size. The RAP is typically mixed with the heated virgin aggregate in the drum mixer at a point prior to adding the liquid asphalt and mineral fines.
The asphalt industry has traditionally faced many environmental challenges. The drum mixer characteristically generates, as by-products, a gaseous hydrocarbon emission (known as blue smoke), various nitrogen oxides (NOx) and sticky dust particles covered with asphalt. Early asphalt plants exposed the liquid asphalt or RAP material to excessive temperatures within the drum mixer or put the materials in close proximity with the burner flame which caused serious product degradation. Health and safety hazards resulted from the substantial air pollution control problems due to the blue-smoke produced when hydrocarbon constituents in the asphalt are driven off and released into the atmosphere. The exhaust gases of the asphalt plant are fed to air pollution control equipment, typically a baghouse. Within the baghouse, the blue-smoke condenses on the filter bags and the asphalt-covered dust particles stick to and plug-up the filter bags, thereby presenting a serious fire hazard and reducing filter efficiency and useful life. Significant investments and efforts were previously made by the industry in attempting to control blue-smoke emissions attributed to hydrocarbon volatile gases and particulates from both the liquid asphalt and recycle material.
The earlier environmental problems were further exacerbated by the processing technique standard in the industry which required the asphalt ingredients with the drum mixer to flow in the same direction (i.e., co-current flow) as the hot gases for heating and drying the aggregate. Thus, the asphalt component of recycle material and liquid asphalt itself came in direct contact with the hot gas stream and, in some instances, even the burner flame itself
Many of the earlier problems experienced by asphalt plants were solved with the development of modern day counter-flow technology as disclosed in my earlier patent Hawkins U.S. Pat. No. 4,787,938 which is incorporated herein by reference and which was first commercially introduced by Standard Havens, Inc. in 1986. The asphalt industry began to standardize on the counter-flow processing technique in which the ingredients of the asphaltic composition and the hot gas stream flow through a single, rotating drum mixer in opposite directions. Combustion equipment extends into the drum mixer to generate the hot gas stream at an intermediate point within the drum mixer. Accordingly, the drum mixer includes three zones. From the end of the drum where the virgin aggregate feeds, the three zones include a drying/heating zone to dry and heat virgin aggregate, a combustion zone to generate a hot gas stream for the drying/heating zone, and a mixing zone to mix hot aggregate, recycle material and liquid asphalt to produce an asphaltic composition for discharge from the lower end of the drum mixer.
Not only did the counter-flow process with its three zones vastly improve heat transfer characteristics, more importantly it provided a process in which the liquid asphalt and recycle material were isolated from the burner flame and the hot gas stream generated by the combustion equipment. Counter-flow operation represented a solution to the vexing problem of blue-smoke and all the health and safety hazards associated with blue-smoke.
A more complete understanding of the early equipment and processing techniques used by the asphalt industry can be found in the extensive listing of prior art patents and printed publications contained in my earlier patents Hawkins U.S. Pat. No. 5,364,182 issued Nov. 15, 1994, Hawkins U.S. Pat. Nos. 5,470,146 issued Nov. 28, 1995, and Hawkins 5,664,881 issued Sep. 9, 1997. Indeed, as a result of my first patent Hawkins 4,787,938 becoming involved in protracted litigation, the prior art collection cited in the foregoing patents is thought to be a thorough and exhaustive bibliographic listing of asphalt technology and such prior art is specifically incorporated herein by reference.
With many of the health and safety issues associated with asphalt production solved by the advent of counter-flow technology, contemporaneous attention has now shifted to operational inefficiencies which are manifest as excessive design and production costs and poor economy of operation from excess energy consumption.
Experience has shown that the environmentally desirable use of a recycled material (RAP) in asphalt production comes with disadvantageous tradeoffs in energy consumption. The most energy efficient plant operation is achieved when no RAP is added. In such circumstances, for example, all virgin aggregate is introduced in one end of the dryer and flows as a falling curtain or veil of material in counter-current heat exchange with hot gases generated at the opposite end of the dryer. The shell temperature is characteristically about 500xc2x0 F. and the exhaust gas is about 225xc2x0 F. which is within the normal operating temperature for the baghouse used to filter the exhaust gas of particulate matter. The temperature of the exhaust gas stream is determined by the design of the dryer, but must be kept above dew pont to prevent moisture from condensing in the exhaust ductwork and especially in the baghouse itself. A temperature of 225xc2x0 F. is sufficient, but since varying conditions during operation can cause relatively large temperature swings, most operations are controlled to keep exhaust temperatures in the range of 250xc2x0 F. to 275xc2x0 F.
The addition of RAP material has a significant effect on operating temperatures of the process. Since RAP cannot be directly dried without burning the liquid asphalt and causing hydrocarbon smoke emissions, it is dried indirectly by superheating the virgin aggregates and then mixing the superheated aggregates with the RAP to achieve a mixed mixture temperature. This results in much higher exhaust gas temperatures and a resulting loss in fuel efficiency. Accordingly, 20 TO 40% RAP feeds (that is, operations wherein RAP makes up 20 to 40% of the final asphalt composition) have been close to the upper end of the range heretofore workable in modem counter-flow asphalt plants. Although a 50% RAP feed has been achievable, it has been at the cost of high energy and reduced equipment life. Consequently, an upper limit of approximately 40% RAP has been a realistic upper limit for the majority of asphalt plants. The operating conditions necessary are illustrative of the problems. If 50% RAP is introduced midstream in the process, then only 50% virgin aggregates are used. This means that only half the material is present, as compared to the 100% virgin aggregate production, to be heated and only half the veiling of material in the drying section of the drum occurs which yields poor heat transfer characteristics. Under such circumstances, the combustion zone temperature must be elevated significantly to superheat the virgin aggregate. This, in turn, causes the shell temperature of the drum to range from 750-800xc2x0 F. and the exhaust gas temperature to increase to about 375xc2x0 F. The exhaust gas temperature will now exceed the upper limit for a baghouse using polyester bags which have an upper service of about 275xc2x0 F. Accordingly, more costly filter bags constructed of less heat sensitive material such as NOMEX (an aramid fiber marketed by DuPont) have to be installed in the baghouse whenever higher RAP feed operations are contemplated. Moreover, any time the combustion zone temperature rises to about 2800xc2x0 F. or greater then the production of various nitrogen oxides (NOx, as a product of combustion becomes a problem.
A need remains in the industry for an improved counter-flow asphalt plant design capable of utilizing high percentage RAP mixes and for operating techniques to address the problems and drawbacks heretofore experienced with modem counter-flow production. The primary objective of this invention is to meet this need.
More specifically, an object of the invention is to provide a counter-flow asphalt plant capable of routinely using high percentage RAP mixes (e.g., up to 50% RAP) without emitting excessive blue smoke or without excessive energy requirements.
Another object of the invention is to provide a counter-flow asphalt plant capable of processing up to 50% RAP mixes with extended equipment life by eliminating the need to superheat virgin aggregates with the associated temperature elevation of the processing equipment.
An alternative object of the invention is to provide a counter-flow asphalt plant capable of processing RAP mixes greater than 50% by utilizing superheating techniques together with the processing techniques which are the subject of this invention.
An additional object of the invention is to provide a counter-flow batch plant dryer for processing up to 50% RAP mixes without emitting excessive blue smoke.
Another object of the invention is to provide counter-flow drum mixer or batch plant dryer equipment and method of operation for retrofitting existing asphalt plants to increase production capacity by reducing the total volume and temperature of the combustion gases present in the equipment for a given production rate.
A corollary object of the invention is to provide counter-flow drum mixer or batch plant dryer equipment and method of operation of the character previously described for retrofitting existing asphalt plants to increase production capacity by as much as 20%.
An additional object of the invention is to provide counter-flow drum mixer or batch plant dryer equipment of a reduced size for a given production rate for savings in original equipment costs, as well as savings in operating costs, by reducing the total volume and temperature of the combustion gases necessary to achieve a given production rate in a conventional counter-flow plant.
A corollary object of the invention is to provide counter-flow drum mixer or batch plant dryer equipment and method of operation of the character previously described that reduces by as much as 20% the size of the equipment required to produce a given volume of product.
A further object of the invention is to provide a counter-flow drum mixer or batch plant dryer equipment with specially designed combustion zone flighting to permit RAP material to be introduced into the drum between the ends of the combustion zone.
Another object of the invention is to provide equipment and methods of operation to permit RAP material to be introduced into an asphalt plant drum between the ends of the combustion zone, to be shielded from direct radiant heat to minimize blue smoke production, and to permit any blue smoke which is formed to be incinerated in the remainder of the combustion zone.
Yet another object of the invention is to provide counter-flow drum mixer or batch plant dryer equipment and method of operation for reducing NOx emissions for processing techniques utilizing both virgin material mixes and RAP with virgin material mixes.
An additional object of the invention is to provide counter-flow drum mixer or batch plant dryer equipment and method of operation which both reduces in size and operates more economically the air handling equipment and dust collection system required for asphalt production.
Another object of the invention is to provide counter-flow drum mixer or batch plant dryer equipment and method of operation for which the exhaust gas temperatures are substantially lower than in conventional systems (225 F. average vs. 375 F. average in a typical 50% recycle plant) to permit the use of polyester filters in the dust collection system for a savings of 80% in filter cost over conventional systems.
A further object of the invention is to provide a counter-flow asphalt plant of the character described having improved efficiency of operation and production consistency of finished product conforming to specifications.
An additional object of the invention is to provide a counter-flow asphalt plant of the character described having more precise control over operating parameters to achieve a uniform end-product and more precise control over energy requirements for improved economic operation.
An added object of the invention is to provide a counter-flow asphalt plant of the character described which meets or exceeds modem day environmental standards.
A further object of the invention is to provide a counter-flow asphalt plant of the character described which is both safe and economical in operation. Efficient operation results in improved fuel consumption and in reduced air pollution emissions.
Other and further objects of the invention, together with the features of novelty appurtenant thereto, will appear in the detailed description of the drawings.
In summary, a counter-flow aggregate dryer for an asphalt plant is equipped with a secondary feeder for introducing RAP or virgin materials intermediate the ends of the combustion zone of the dryer. Nonveiling flights in the combustion zone shield material carried through the combustion zone from direct radiant heat and veiling flights in the drying zone create a curtain of falling aggregates heated by a hot gas stream flowing in a countercurrent direction from a primary burner. A secondary burner elevates the temperature of the exhaust gas above its dew point temperature before delivery to the baghouse.